1. Field of the Invention
The present invention relates to methods of radioactively labeling diagnostic and therapeutic agents and is particularly related to systems in which a metal ion is bound to the labeled molecule through a chelating group.
2. Description of the Prior Art
The use of radioactively labeled diagnostic and therapeutic agents has become routine practice in clinical and analytical laboratories throughout the world. Such radioactively labeled compounds are used both in vitro (for example, in radioimmunoassay systems) and in vivo (for example, both in diagnostic imaging techniques and in radiation therapy techniques).
Initially, the number of radioisotopes that could be firmly attached to the typical organic molecules used as diagnostic and therapeutic agents was limited. The difficulty in forming stable carbon-metal bonds prevented the early utilization of many radioactive metals and typically limited radioisotopes used to label organic molecules to isotopes of phosphorus, carbon, hydrogen, and iodine.
Recently, a new approach has enabled the labeling of such agents with metal ions. In this approach, a chelating moiety is covalently attached to the molecules of interest, and a radioactive ion is then chelated by the sequestering groups of the chelator. The chelating moieties which have generally been used for this purpose in the prior art have been analogues or derivatives of ethylenediaminetetraacetic acid (EDTA), although many variations have also occurred. For example, in 1968, W. F. Benisek and F. M. Richards suggested the covalent bonding of chelating groups based on methylpicotinimidate to the amino function of a protein molecule in order to facilitate crystallographic investigation of protein structure by binding a metal to the chelating site on the protein [J. Biol. Chem., 243 4267-4271 (1968)]. Likewise, in U.S. Pat. No. 4,043,998, the compound 1-(p-benzenediazonium)ethylenediaminetetraacetic acid, said to be a powerful chelating agent which can be bonded strongly to proteins through its diazonium group, was disclosed. In Science, 209, 295-297 (1980), B. A. Khaw et al. disclosed the use ofa bifunctional chelating agent, diethylenetriaminepentaacetic acid (DTPA) to label an antibody with a radioactive isotope and the subsequent use of that labeled antibody to image experimental myocardial infarctions in dogs. The metal binding efficiencies of the resulting compounds were low, however, since attachment occurred through one of the carboxylate groups which would normally have participated in binding to the metal ion. Similarly, D. A. Scheinberg and O. A. Gansow taught in Science, 215 1511-1513 (1982), the use of DTPA and EDTA analogs covalently bonded to antibodies to image mouse erythroid tumors.
Unfortunately, the radioactively labeled materials previously available suffered from several disadvantages. This was particularly true for imaging agents and other molecules labeled with an isotope of high specific activity. The short half-lives of the radioactive isotopes used and the radiation-induced degradation of the labeled molecules greatly reduced the shelf-lives of these materials and, when imaging agents are involved, greatly increased the amount of background radiation present. Furthermore, health hazards to the technicians handling these materials and hazards associated with disposing of the associated waste generated at various steps of synthesizing labeled compounds made the handling of radioactively labeled compounds difficult.
Typically, as disclosed by Scheinberg and Strand in the article cited above, a bifunctional chelate was coupled to a target molecule, after which any metal ions present were removed by dialysis, typically against a solution containing low molecular weight chelating molecules such as EDTA. The chelate-conjugated molecules were then labeled with a radioactive metal solution, after which free metal was removed, for example, by ion-exchange chromatrography. The resulting labeled product was then stored and later used in the diagnostic or therapeutic process. Using such procedures, considerable handling of the radioactive material and generation of radioactive waste occurred, a disadvantage not overcome by any teachings of the prior art.